Process for densifying felts



be continued for -12 hours or more. required for particular applications, for example for pol- Patented June 6, 1961 A non-exclusive, irrevocable, royalty-freelicense in the invention herein described, throughout the world for all purposes of the United States Government with the power to grant sublicenses for such purposes, is hereby granted to the Government of the United States of America.

This invention relates to and has as its primary object .the provision of novel methods for increasing the density and imparting other desirable changes in the properties of wool-containing felts by chemical treatment.

In the conventional preparation of felts, webs of carded wool, with or without other fibers, are depositedone over the other to form loose batts. These batts are moistened,

steamed, treated with a lubricant .such as soap, and subjected to vibratory pressure to compact the batts. The compacted batts are then subjected torepeated pounding, rolling, and kneading to interlock the fibers into a dense sheet form. As this mechanical action, generally referred to as fulling, is continued the felt becomes increasingly dense. In preparing soft felts the mechanical-action may beapplied for relatively short times of about 15 minutes; for preparing hard sheet felts the mechanical action must Hard felts are often ishing wheels; artificial limbs; cushioning pads for looms, presses, and other machinery; clutches; brakes, .etc.

In accordance with the invention, felts are hardened or densified by a chemical action instead of mechanical action. A particular advantage of the process of the invention is that elimination of the extended mechanical treatment results in a substantial saving in-operating costs. Another advantage is that the time required to achieve a desired degree of hardness is less than with mechanical treatment. Moreover, the process of the invention requires no specialized equipment and is outstanding in its simplicity and effectiveness.

The objects of the present invention are attained by impregnating a felt with dimethyl sulfoxide and applying heat. .The heating is continued until the felt develops the desired degree of hardness.

When the felt is heated in the presence of dimethyl sulfoxide, the individual wool fibers contract or shrink, this causing the entire mass of fibers to occupy a decreased volume, thus forming a denser mass. Where-the felt contains other fibers in additionto wool, theshrinkage of the wool fibers naturally draws these fibers together since all the fibers of the mass :aremattedandentangled. Generally, under any specified conditions of a temperature, time, etc. a greater densification will .occur with felts of greater proportion of wool fibers. Although the theory of the process is imperfectly understood, it has been determined that there is no significant increase or decrease in weight as a result of the process of the invention. it is therefore believed that the dimethyl sulfoxide acts as a catalyst :in promoting a chemical rearrangement, rather than adding to the wool Emolecules. Thus certain chemical bonds in the wool molecule are disrupted and then re-formed into new patterns; as a result a the individual fibers contract.

Although certain chemical .-changes are believed to take place, it is to be noted that .as immersion, spraying,,orby the useof lick rolls.

comes increasingly .dense.

As noted above, the process of the invention results in increasing the density of the original felt. :Depending on the conditions applied, the decrease in ,volume maybe as low as 57% or as high as 53%. In addition to increased density, the products exhibit increase in'hardness, splitting resistance, abrasion resistance, ultimate elongation, and tensile strength. These desirable changes are believed to be caused by the increased degree of fiber "interlocking in the-treated-felts.

It has been observed that when commercial felts are hardened by the process of the invention, the dimensional decrease is mainly in the length and width of the sheet. There is relatively little change in thickness. This is believed to be due to the way the felts are ordinarily made by superimposing webs of carded fibers. As a result there is a lesser degree of interlocking of fibers in a plane normal to the surfaceo'f the sheet than in planes parallel to the surface. -If it is desired to attain a greaterreduction in thickness during the dimethyl sulfoxide treatment, the felts may be given 'apre-treatment to attain a greater degree of interlocking of fibers in the plane normal to thebarbed needles through the sheet.

In the one modification of the invention, the felt to be 'hardened'is :first .impregnated with dimethyl sulfoxide, .then heated. The dimethylzsulfoxide may be applied to the felt as such or dissolved in a volatile inert solvent such as water, methanol, ethanol,propanol, isopropyl alcohol, etc. Application of the compound (or solution thereof) to the felt may be by any conventional technique Since dimethyl sulfoxide acts as a catalyst rather than as a reagent, the proportion applied to the felt is not critical. For example, the proportion of dimethyl sulfoxide may be as low as 10% based on-the weight of the felt. Usually, however, a larger proportion of the compound is used .to compensate for loss of the compound by vaporization during the heating step. The use of excessive amounts of dimethyl sulfoxide does not do any harm and theresidue in the product is easily removed after heating by extraction.

in the heating step the dimethyl sulfoxide-impregnated felt is heated at a temperature which may range from oven, by subjection to heated platens, by:irradia-tion with infra-red rays, etc. The time of heating will -vary depending on the conditions used, particularly temperature, and on the degree of densification desired. peratures are conducive toincreased rate of densification. Also, as the heat treatment is continued the product be- Heating times may be anywhere from 5 minutes to 12 hours depending on .theternperature selected and the degree of densification'desired. To prevent loss of dimethyl sulfoxide by vaporization, the felt may be heated while contained in a sealed vessel such as a retort, autoclave, or the like.

Although it is generally preferred to carryout the process of the invention by first applying. dimethylfsulfoxide to the felt and thenjheating, "the dimethyl snljfoxide may be applied simultaneously with -heating. 'T'husfor example, the Tfelt may i'be immersed :into ;a tprroliiif dimethyl sulfoxide maintained at the aforesaid temperature range (-150 C.). The felt is held therein until the desired degree of densification is attained. Another plan is to expose the felt to hot vapors of dimethyl sulfoxide. Thus the felt may be suspended in a vessel contaiping at the base a suitable quantity of dimethyl .s V By applying heat to the base of the vessel, 'thedi ethyl sulfoxide is vaporized and so distributed into therintervstices-of :the felt. The .vesselnsed in this procedure (may be a pressure-tight autoclave, .in which case the system Higher tem- 3 may be maintained at a pressure lower than atmospheric so that the dimethyl sulfoxide will boil at a temperature within the desired range of 85l50 C. At normal pressure dimethyl sulfoxide boils at 190 C.

After the felt has been heated in the presence of dimethyl sulfoxide for a period suflicient to attain the desired increase in density, any residual dimethyl sulfoxide may be removed by extracting the felt with water, acetone, methanol, ethanol, propanol, isopropyl alcohol, or other volatile solvent for the compound.

The process of the invention may be applied to allwool or part-wool felts. Generically then, the invention is applicable to wool-containing felts, by which expression is meant felts containing at least 20% by weight of wool. The remaining fibers in the felt may be any natural or artificial textile fibers which are insoluble in dimethyl sulfoxide. Thus the non-wool components of the felt may be cotton; kapok; linen; natural silk; regenerated cellulose fibers produced by the viscose, cuprammonium, or nitro-cellulose process; jute; hemp; animal hair; glass fibers; asbestos, etc.

The invention may be utilized in various ways. For example, commercial grades of felts may be hardened to any desired density by application of the process of the invention. Another plan is to use the chemical process of the invention as a substitute for all or part of the conventional mechanical fulling action. Thus for example batts of fibers may be subjected to fulling only to the extent of forming a soft felt and then the process of the invention is applied to form a hard felt therefrom. Another field of application is in the production of felts of a hardness not attainable by mechanical treatment. Thus a hard felt produced by conventional fuliing action may be subjected to the process of the invention to produce extremely hard felts of hitherto unattainable density.

The invention is further demonstrated by the following illustrative examples.

Example I A commercial mechanical roll wool felt 0.7 cm. thick was cut into a strip 10.1 cm. wide and 19.1 cm. long. The strip was placed in the bottom of an enamelware tray and dimethyl sulfoxide was poured into the tray to just cover the felt strip. The tray was heated for 3 hours in an oven maintained at 105 C. After this period the felt was removed from the tray, extracted with acetone to remove excess dimethyl sulfoxide, and dried in air. It was observed that the treated strip had these dimensions: width 7.8 cm., length 15.1 cm., and thickness 0.6 cm. There was no significant change in the weight of the treated strip.

Samples of the untreated and treated felt were then subjected to various tests to determine the change in properties caused by the dimethyl sulfoxide.

The apparent specific gravity was obtained by the formulawhere A=apparent specific gravity G=weight of specimen in grams V=volume of specimen in cc.

The compactness was derived by the formula A O=m 100 where c=compactness (percent) 1.32=specific gravity of wool The compactness (sometimes referred to as bulk dencity or percent specific gravity) is an index of the proportion-of the total volume of the felt which is occupied 4 by fibers. For example, a compactness of 25% signifies that of a sample of felt, one-quarter of the total volume is occupied by fibers, the remainder is, of course, air.

The results are tabulated below:

Apparent Com- Sample specific paetuess,

gravity, percent g./cc.

Untreated 0. 37 28 Treated 0. 70 53 The compression properties of the untreated and treated felts were then tested by subjecting them to pressure and measuring the decrease in thickness under increasing compression loads. The results are tabulated below:

Example II A sample of S.A.E. F-lS felt (wool content remainder animal hair and cotton), 4" thick, was cut into strips 7" long and 2" Wide. These strips were treated with dimethyl sulfoxide as described in Example I, except that in this case the heating was for 5 hours. After the treatment, the dimensions of the strips were: length 5%", width 19%", thickness A". There was no significant change in the weight of the treated strips.

Specimens of the treated and untreated felt were tested for splitting resistance by the method ASTM D46l-53. The results are tabulated below:

Splitting resist- Sample ance for 2" strip, lbs.

Untreated 2. 29 Treated 5. 89

Example III 'lhick- Reduction Sample Width, Length, ness, in volume. cm. cm. em. percent oi original Untreated 5 27 0. 8 Treated (1 hr. heatlng). 5 25 0. 8 7. 4 Treated (4 hrs. heating)- 4 20 0. 8 40. 7

The treated and untreated samples were then tested for elongation by the method of ASTM D461-53. In this test the samples are subjected to increasing tension until the sample breaks and the elongation attained at this point is measured. The results are tabulated below.

Elongation Sample at break,

percent Untreated 53 Treated 51 hr. heating) 64 Treated 4 hrs. heating) 87 Example IV A sample of S.A.E. F-15 felt (wool content 90%, remainder animal hair and cotton), A thick, was cut into a circular disc 41 mm. in diameter. The disc was immersed for a period of minutes in dimethyl sulfoxide which had been preheated to 140 C. The felt disc was then extracted with acetone and dried in air. The treated disc was 28 mm. in diameter, corresponding to a reduction in volume of 53%.

Example V Another disc was cut from the felt of Example IV. Its diameter was 41 mm. and its weight was 1.7 g. Three tenths of one milliliter of dimethyl sulfoxide was placed in a 100-ml. beaker. The felt disc was hung by a wire so that it was horizontally suspended /2 inch above the liquid surface. The beaker was heated at the boiling point of dimethyl sulfoxide, 190 C., and the disc was kept exposed to the hot vapors for 10 minutes. This treatment caused the felt to shrink so that there was a 37% reduction in volume.

Example VI comprises heating a wool-containing felt impregnated with dimethyl sulfoxide until its density is substantially increased.

2. A process for increasing the density of felts which comprises impregnating a wool-containing felt with dimethyl sulfoxide and heating it until the desired degree of density is attained.

3. A process for increasing the density of felts without application of mechanical action which comprises impregnating a wool-containing felt with dimethyl sulfoxide and heating it until its density is increased.

4. A process of producing hardened felts which comprises applying mechanical fulling action to a mass of textile fibers including wool fibers until a felt of predetermined hardness is produced, discontinuing the mechanical action, impregnating the felt with dimethyl sulfoxide and heating it until a hardened felt is formed.

5. A process of producing hard felts with a minimum application of mechanical action which comprises applying mechanical fulling action to a mass of textile fibers including wool fibers only until a soft felt is produced, discontinuing the mechanical action, impregnating the soft felt with dimethyl sulfoxide and heating it until a hard felt is formed.

6. A process of producing felts of a hardness unattainable by mechanical action which comprises applying mechanical fulling action to a mass of textile fibers including wool fibers until substantially no further increase in hardness is attained, discontinuing the mechanical action, impregnating the resulting felt with dimethyl sulfoxide and heating it until a felt of increased hardness is formed.

References Cited in the file of this patent FOREIGN PATENTS Great Britain Mar. 24, 1947 OTHER REFERENCES 

6. A PROCESS OF PRODUCING FELTS OF A HARDNESS UNATTAINABLE BY MECHANICAL ACTION WHICH COMPRISES APPLYING MECHANICAL FULLING ACTION TO A MASS OF TEXTILE FIBERS INCLUDING WOOL FIBERS UNTIL SUBSTANTIALLY NO FURTHER INCREASE IN HARDNESS IS ATTAINED, DISCONTINUING THE MECHANICAL ACTION, IMPREGNATING THE RESULTING FELT WITH DIMETHYL SULFOXIDE AND HEATING IT UNTIL A FELT OF INCREASED HARDNESS IS FORMED. 